1. Field of the Invention
The subject of the present invention is a method for manufacturing a semiconductor pressure sensor.
2. Description of the Related Art
As is known, a pressure sensor is a device that converts a variation in pressure into a variation of an electrical quantity (a resistance or a capacitance). In the case of a semiconductor sensor, the pressure variation is detected by a membrane of semiconductor material, which overlies a cavity and is able to undergo deflection under mechanical stress.
Pressure sensors using semiconductor technology typically find their application in medicine, in household appliances, in consumer electronics (cell-phones, PDAs—Personal Digital Assistants—), and in the automotive field. In particular, in the latter sector, pressure sensors are used traditionally for detecting the pressure of the tyres of motor vehicles, and are used by the control unit for alarm signaling. Pressure sensors are, on the other hand, also used for monitoring air-bag pressure, for controlling the breakdown pressure of the ABS, and for monitoring the pressure of oil in the engine, the pressure of injection of the fuel, etc.
Currently existing sensors manufactured using the semiconductor technology are basically of two types: piezoresistive and capacitive sensors.
Operation of piezoresistive sensors is based upon piezoresistivity, i.e., the capability of some materials to modify their resistivity as the applied pressure varies. Piezoresistors are normally formed on the edge of a suspended membrane (or diaphragm) and are connected to one another in a Wheatstone-bridge configuration. Application of a pressure causes a deflection of the membrane, which in turn generates a variation in the offset voltage of the bridge. By detecting the voltage variation with an appropriate electronic circuit, it is possible to derive the desired pressure information. An example of a piezoresistive sensor of the above type is described in U.S. Pat. No. 6,131,466.
Sensors of a capacitive type are based upon the change in capacitance that occurs when a pressure is applied on a flexible membrane suspended above a support and separated therefrom by a region that is empty or filled with gas (air gap). Two examples of silicon sensors of a capacitive type are described in “A MEMS-Based, High-Sensitivity Pressure Sensor for Ultraclean Semiconductor Applications”, A. K. Henning, N. Mourlas, S. Metz published on: http://www.redwoodmicro.com/Papers/ASMC.pdf and “Application of High-Performance MEMS Pressure Sensors Based on Dissolved Wafer Process” A. Tadigadapa, S. Massoud-Ansari, published on: http://www.mems-issys.com/pdf/issystech2.pdf.
In the case where the dielectric present between the two electrodes is a vacuum, an absolute pressure sensor is obtained, whereas, if gas is present, which is generally introduced hermetically at a known reference pressure, the detected capacitance variation is linked to the difference between the external pressure and the internal pressure, and, consequently, a relative pressure sensor is obtained.
Application of a pressure causes a deflection of the membrane with consequent reduction in its distance from the bottom electrode. In this way, the capacitance of the pressure sensor increases. By measuring the difference between the capacitance thus obtained and the rest capacitance (i.e., in the absence of stress), the pressure variation detected by the sensor is obtained.
Also in this case, a circuit for processing the electrical signals generated by the capacitive sensor provides the information of pressure sought.
In general, capacitive technology presents a lower current consumption than does piezoresistive technology. Consequently, capacitive sensors are preferable in those applications where power consumption is an important parameter, for example, in the automotive field, wherein accurate control of the load power is required. Moreover, capacitive pressure sensors present smaller overall dimensions and lower costs, as is required in numerous applications.
Both piezoresistive sensors and capacitive sensors hence call for the construction of a cavity underneath the flexible membrane.
Currently, various solutions have been proposed:
1. use of silicon-on-insulator (SOI) substrates; for instance, pressure sensors using this solution are described in U.S. Pat. Nos. 5,369,544; 5,510,276 and 6,131,466;
2. use of porous silicon (see, for example, U.S. Pat. No. 5,242,863);
3. wet etching from the front (see, for example, U.S. Pat. No. 4,766,666);
4. wet etching from the rear, using tetramethyl ammonium hydroxide (TMAH);
5. other methods (see, for example, U.S. Pat. No. 4,744,863).
In all known solutions, the use of semiconductor technology for making cavities underneath suspended structures and layers calls for processes that are complex, costly and, in some cases, far from compatible with the manufacturing steps currently used in the semiconductor industry for manufacturing integrated circuits.